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What is Tufa Stone?


#1

Being relatively new to all this, I have a sort of "dumb"
question. Well, not “Dumb”, but “ignorant”.

What is “tufa stone”? Is there a picture of it somewhere on the
Web?

Thanks, All!

Cheryl


#2
   What is "tufa stone"? Is there a picture of it somewhere on
the Web? 

Tufa is a volcanic ash rock. That used for casting is
fine-grained, with very few large inclusions of other material.
It is also called tuff. I don’t know of a web site, but any good
visual reference geology book should have a picture of it,
description and locations where it can be found.


#3

Tufa stone is compacted volcanic ash,found on the Navajo
reservation. Used to be you could get it in various
densities,don’t know about now. I buy mine from Indian Jewelers
Supply in Gallup,New Mexico, and buy only what they sell. If
anyone would like to know more about its original use(when first
used by the Navajo) there is a book that is now back in print
entitled “The Navajo and Pueblo Silversmiths”, by John Adair.
Written in 1944. The book is a complete history of native
American silversmithing. It is now put out by The University Of
Oklahoma Press. Good reading…John Barton


#4
   Do you have any good techniques for using Tufa ?    I have
a few big pieces laying around in my studio and would love to
use it... 

Native American “sandcast” pieces, usually featuring najas (the
main horseshoe shape of the squash blossom necklace), ketos
(bowguards), buckles and sandcast bracelets began their lives as
tufa carvings. These designs were heavily influenced by the
Spanish who used them as forestall and headstall decorations on
their horses. A carving is made in tufa stone and cast. This
becomes the master model for casting in sand or cement.

Because of the nature of casting in tufa, which has to rely on
the force of gravity alone to get the molten metal to all areas
of the design, you have to plan your design with this in mind.
Most of these pieces are visually and physically massive. You
must plan for sizeable bridges between each part of the design.
This is why you’ll often see the design in either half-round or
triangular shapes for the pattern. Think of designing with
either half-round or triangular wire.

You need two blocks of tufa for each carving. You will carve a
bas-relief in the negative on one half. The other half is the
flat back. Both blocks need to have the sprue hole cut into them
however, half of the sprue on each block.

After both blocks have been trued, lay out your pattern you
intend to carve on one block. Leave about 1/2" uncarved stone for
very small castings, proportionately larger for larger carvings.
Often, I just use a pencil to draw directly onto the stone. My
favorite tools for carving the tufa are my wax tool (which
dentists call a carver), a scooped spatula wax tool and a small
paring knife. After the carving is done, check your bridges, add
sprue and air vents.

Next stabilize the carving with aqueous sodium silicate which
has been thinned out with water (50/50 ratio) by painting it on
all sides and allow it to dry thoroughly. This stabilization
process can also be used to preserve your cuttlefish carvings.
This makes it hard so you can use it for several castings, but
the first casting should always be saved for your master. This is
usually cast in bronze, which is then used to make sand castings
after it’s been cleaned up.

Soot up the casting sides of both blocks generously. Use a brush
to remove any excess soot. The soot allows you to remove the
casting without it sticking to the stone and pulling out your
carved design, but any loose soot can be carried into the casting
and ruin it. Bind the two halves together with wire and cast
away.

Until you get the hang of how large your bridges need to be, the
depth of the carving and so-on, start with small designs. You’ll
get the idea very quickly and will then be able to progress to
larger patterns.

I hope this helped to answer your questions and you’ll start
using those pieces of tufa laying around. KP in Wyoming


#5

Thank you, Kathy, for the on using Tufa and for the
tip on preserving cuttlefish molds for multiple castings! I use
cuttlefish a fair amount to add interesting texture to my pieces,
mixing fabricated high and low polish, stippled, etc, with areas
of cuttlefish cast but have always had to dispose of it when I
was done with just one cast. I am confessing ignorance - where
do I find “sodium silicate”? Is there another name by which I
might know it? Thanks!

Shael Barger
dakotahdog@msn.com


#6

Cheryl,

Tufa is a soft easily carved stone which has been used many
years by native silversmiths as a mold material for casting
silver. I’ve never used it, living too far east to obtain it
readily. But my interest was piqued when I tried cuttlebone
casting recently. From what I have read tufa is reusable a few
times even when a high temperature metal like silver is cast.
Cuttlebone is only good for 2 or 3 casts if using pewter or other
low temp metal and one cast if using silver. Cuttlebone is
fairly easily obtainable at bird shows. I got a big bag,
containing about 100 cuttlebones for $20 a couple of years and so
far haven’t used all them. Cuttlebone will not cast intricate
designs like investment plaster. I am hoping to get some tufa and
see what I can do with it. It it turns out I’ll post some results
later on.

All the others who posted, thanks. This should prove really interesting.

Georgie


#7

Sodium silicate is also known as “water glass”. You can often
find it in drug stores and such places. margaret
@Margaret_Malm


#8

Shael, sodium silicate (“water glass”) may also be obtained from
a ceramics supply place. Rene Roberts